Local Coupled Cluster response for open-shell systems

• Applicant: Professor Dr. Martin Schütz (†)
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
• Term: from 2011 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 202588754

The calculation of properties of electronically excited states of large molecules is of great importance for many applications in physics, chemistry, and biology. In the previous project we successfully implemented analytical nuclear energy gradients for ground and excited singlet and triplet states of closed-shell molecules at the level of local CC2 response. This allows for excited state geometry optimizations at that level of theory for systems of the size of chlorophyllide-a. Furthermore, we devised a hierarchy of efficient coupled cluster response methods for calculating ionization potentials (IPs).The goal of this project continuation is the extension of this methodology to open shell systems. In particular we aim at excited states of large radicals. These often play an important role as intermediates in photochemical and photocatalytical processes and can be detected by means of spectroscopical methods. In the project we intend to approach this goal from two sides, i.e., (i) by traditional spin orbital based open-shell coupled cluster response theory, and (ii) by coupled response theory for IPs. In the latter case the excitation energies of the radical are computed as differences of two IPs. The reference wavefunction is that of the related closed-shell molecule with N+1 electrons, and the ionized states representing the radical are obtained as pure doublet states (without any spin contamination).We intend to implement excitation energies, as well as transition strengths, first-order properties, and analytical nuclear gradients for ground and excited states of radicals. Local approximations and density fitting will be utilized to dramatically reduce the computational cost and, in this way, to make the methods applicable to large systems. We think that the programs developed in the course of this project will be very useful to study radical intermediates in photochemical and photocatalytical reactions.

• DFG Programme: Research Grants